A vertical transport field effect transistor (VTFET) is a transistor in which the current between the drain and source electrodes is primarily normal to the top surface of the die. That is, a VTFET device is a device in which the current flow between the source and drain regions of the device is primarily orthogonal to the primary surface of the substrate. For example, in a VTFET device, the source and drain are provided at a top and bottom portion of the fin, with the channel along a vertical extent of the fin between the source and drain regions. The VTFET is advantageous in that it allows a more dense and integrated usage of FETs in the active area of the chip, which leads device scaling.
The bottom source region or drain region in VTFETs are formed very early in the flow process. Due to this, the bottom source or drain region see significant thermal steps from downstream process flow. This leads to dopant up-diffusion into the channel which can negatively impact device performance. For example, the dopant up-diffusion can negatively impact mobility, threshold voltage (Vt), short channel effect (SCE) and off-state current (Ioff).